1. Field of the Invention
This invention relates to an optical element whose focal length is variable.
2. Description of the Prior Art
Lenses, mirrors, gradient index type light H) transmitters, etc. are well-known as optical elements for imparting a converging action or a diverging action to an incident light beam. To make the focal lengths of these optical elements variable, one of the radius of curvature, the refractive index and the thickness of the lens, the mirror or the like must be varied. However, in the lens, the mirror or the like, once the shape and material thereof are determined, the refractive index thereof is primarily determined and the radius of curvature and thickness thereof cannot be varied unless the lens, the mirror or the like is re-worked.
In recent years, elements using an electro-optic material or liquid crystal have been proposed as elements capable of effecting convergence and divergence of light, but these elements depend on the polarization of light and accordingly, the usages thereof are limited.